X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=docs%2FVectorizers.rst;h=2b702179bf283e497f6b0436af232d697d54e62d;hb=26d628d6ce20548dcb7c99a9519b9034bbf9f5e7;hp=0894b1eb3f10f3f3ac4bd26c9896937110945764;hpb=abafaba4d546352d37f8ccae895b0234b6c735f2;p=oota-llvm.git diff --git a/docs/Vectorizers.rst b/docs/Vectorizers.rst index 0894b1eb3f1..2b702179bf2 100644 --- a/docs/Vectorizers.rst +++ b/docs/Vectorizers.rst @@ -6,12 +6,14 @@ Auto-Vectorization in LLVM :local: LLVM has two vectorizers: The :ref:`Loop Vectorizer `, -which operates on Loops, and the :ref:`Basic Block Vectorizer -`, which optimizes straight-line code. These vectorizers +which operates on Loops, and the :ref:`SLP Vectorizer +`. These vectorizers focus on different optimization opportunities and use different techniques. -The BB vectorizer merges multiple scalars that are found in the code into -vectors while the Loop Vectorizer widens instructions in the original loop -to operate on multiple consecutive loop iterations. +The SLP vectorizer merges multiple scalars that are found in the code into +vectors while the Loop Vectorizer widens instructions in loops +to operate on multiple consecutive iterations. + +Both the Loop Vectorizer and the SLP Vectorizer are enabled by default. .. _loop-vectorizer: @@ -21,19 +23,12 @@ The Loop Vectorizer Usage ----- -LLVM's Loop Vectorizer is now available and will be useful for many people. -It is not enabled by default, but can be enabled through clang using the -command line flag: +The Loop Vectorizer is enabled by default, but it can be disabled +through clang using the command line flag: .. code-block:: console - $ clang -fvectorize -O3 file.c - -If the ``-fvectorize`` flag is used then the loop vectorizer will be enabled -when running with ``-O3``, ``-O2``. When ``-Os`` is used, the loop vectorizer -will only vectorize loops that do not require a major increase in code size. - -We plan to enable the Loop Vectorizer by default as part of the LLVM 3.3 release. + $ clang ... -fno-vectorize file.c Command line flags ^^^^^^^^^^^^^^^^^^ @@ -56,6 +51,89 @@ Users can control the unroll factor using the command line flag "-force-vector-u $ clang -mllvm -force-vector-unroll=2 ... $ opt -loop-vectorize -force-vector-unroll=2 ... +Pragma loop hint directives +^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The ``#pragma clang loop`` directive allows loop vectorization hints to be +specified for the subsequent for, while, do-while, or c++11 range-based for +loop. The directive allows vectorization and interleaving to be enabled or +disabled. Vector width as well as interleave count can also be manually +specified. The following example explicitly enables vectorization and +interleaving: + +.. code-block:: c++ + + #pragma clang loop vectorize(enable) interleave(enable) + while(...) { + ... + } + +The following example implicitly enables vectorization and interleaving by +specifying a vector width and interleaving count: + +.. code-block:: c++ + + #pragma clang loop vectorize_width(2) interleave_count(2) + for(...) { + ... + } + +See the Clang +`language extensions +`_ +for details. + +Diagnostics +----------- + +Many loops cannot be vectorized including loops with complicated control flow, +unvectorizable types, and unvectorizable calls. The loop vectorizer generates +optimization remarks which can be queried using command line options to identify +and diagnose loops that are skipped by the loop-vectorizer. + +Optimization remarks are enabled using: + +``-Rpass=loop-vectorize`` identifies loops that were successfully vectorized. + +``-Rpass-missed=loop-vectorize`` identifies loops that failed vectorization and +indicates if vectorization was specified. + +``-Rpass-analysis=loop-vectorize`` identifies the statements that caused +vectorization to fail. + +Consider the following loop: + +.. code-block:: c++ + + #pragma clang loop vectorize(enable) + for (int i = 0; i < Length; i++) { + switch(A[i]) { + case 0: A[i] = i*2; break; + case 1: A[i] = i; break; + default: A[i] = 0; + } + } + +The command line ``-Rpass-missed=loop-vectorized`` prints the remark: + +.. code-block:: console + + no_switch.cpp:4:5: remark: loop not vectorized: vectorization is explicitly enabled [-Rpass-missed=loop-vectorize] + +And the command line ``-Rpass-analysis=loop-vectorize`` indicates that the +switch statement cannot be vectorized. + +.. code-block:: console + + no_switch.cpp:4:5: remark: loop not vectorized: loop contains a switch statement [-Rpass-analysis=loop-vectorize] + switch(A[i]) { + ^ + +To ensure line and column numbers are produced include the command line options +``-gline-tables-only`` and ``-gcolumn-info``. See the Clang `user manual +`_ +for details + Features -------- @@ -187,11 +265,14 @@ that scatter/gathers memory. .. code-block:: c++ - int foo(int *A, int *B, int n, int k) { - for (int i = 0; i < n; ++i) - A[i*7] += B[i*k]; + int foo(int * A, int * B, int n) { + for (intptr_t i = 0; i < n; ++i) + A[i] += B[i * 4]; } +In many situations the cost model will inform LLVM that this is not beneficial +and LLVM will only vectorize such code if forced with "-mllvm -force-vector-width=#". + Vectorization of Mixed Types ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ @@ -245,6 +326,17 @@ See the table below for a list of these functions. | | | fmuladd | +-----+-----+---------+ +The loop vectorizer knows about special instructions on the target and will +vectorize a loop containing a function call that maps to the instructions. For +example, the loop below will be vectorized on Intel x86 if the SSE4.1 roundps +instruction is available. + +.. code-block:: c++ + + void foo(float *f) { + for (int i = 0; i != 1024; ++i) + f[i] = floorf(f[i]); + } Partial unrolling during vectorization ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ @@ -288,29 +380,18 @@ And Linpack-pc with the same configuration. Result is Mflops, higher is better. .. image:: linpack-pc.png -.. _bb-vectorizer: - -The Basic Block Vectorizer -========================== - -Usage ------- - -The Basic Block Vectorizer is not enabled by default, but it can be enabled -through clang using the command line flag: - -.. code-block:: console +.. _slp-vectorizer: - $ clang -fslp-vectorize file.c +The SLP Vectorizer +================== Details ------- -The goal of basic-block vectorization (a.k.a. superword-level parallelism) is -to combine similar independent instructions within simple control-flow regions -into vector instructions. Memory accesses, arithemetic operations, comparison -operations and some math functions can all be vectorized using this technique -(subject to the capabilities of the target architecture). +The goal of SLP vectorization (a.k.a. superword-level parallelism) is +to combine similar independent instructions +into vector instructions. Memory accesses, arithmetic operations, comparison +operations, PHI-nodes, can all be vectorized using this technique. For example, the following function performs very similar operations on its inputs (a1, b1) and (a2, b2). The basic-block vectorizer may combine these @@ -318,10 +399,28 @@ into vector operations. .. code-block:: c++ - int foo(int a1, int a2, int b1, int b2) { - int r1 = a1*(a1 + b1)/b1 + 50*b1/a1; - int r2 = a2*(a2 + b2)/b2 + 50*b2/a2; - return r1 + r2; + void foo(int a1, int a2, int b1, int b2, int *A) { + A[0] = a1*(a1 + b1)/b1 + 50*b1/a1; + A[1] = a2*(a2 + b2)/b2 + 50*b2/a2; } +The SLP-vectorizer processes the code bottom-up, across basic blocks, in search of scalars to combine. + +Usage +------ + +The SLP Vectorizer is enabled by default, but it can be disabled +through clang using the command line flag: + +.. code-block:: console + + $ clang -fno-slp-vectorize file.c + +LLVM has a second basic block vectorization phase +which is more compile-time intensive (The BB vectorizer). This optimization +can be enabled through clang using the command line flag: + +.. code-block:: console + + $ clang -fslp-vectorize-aggressive file.c